The present invention relates to an optical transmission apparatus, and more particularly, to a wavelength division multiplexer (WDM) for multiplexing in one optical fiber a multi-wavelength optical signal correctively and having an optical amplifier for amplifying the multi-wavelength optical signal.
Recently, to transmit a larger volume of data, a wavelength division multiplexing technology to multiplex a plurality of optical signals having a different wavelength has been used. As for an optical transmission apparatus for repeating the optical signals multiplexed in wavelength, for example, an optical add/drop multiplexer (OADM) in which a part of a plurality of wavelengths of data to be transmitted are dropped to receive and a part of the plurality of wavelengths are added to transmit, and an optical cross-connect (OXC) in which a plurality of wavelengths is switched to an arbitrary channel are expected. It is thought that most of optical transmission apparatuses in a long distance core network or metro network in the future will be displaced with such OADM or OXC.
It is necessary for an optical amplifier in an optical network using the OADM or OXC to secure a necessary optical signal to noise ratio (SNR) by amplifying an optical signal lowered due to transmission loss between nodes, as well as by amplifying power of the optical signal lowered due to insertion loss of an optical multiplexer, an optical demultiplexer or an optical switch, and further to amplify the signal so that an optical level falls into an input dynamic range of an optical receiver. As described above, the optical amplifier plays an important role in the optical network using the OADM or OXC, and will be increasingly used in the entire optical network. Therefore, for the optical network's development, it is extremely important to provide a economical optical amplifier.
When for an optical amplifier, for example, a doped fiber is used, pumping light directed from a pump light source into the optical amplifier supplies energy to amplify an optical signal. H. Nakano, S. Sasaki, “Dispersion-Compensator Incorporated Optical Fiber Amplifier”, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 7, No. 6, June 1995, describes a technology that two optical amplifiers connected in series share pumping light emitted from one pump light source. Further, JP-A-6-296056 and FIG. 4 of JP-A-6-164515 disclose that an optical amplifier on the transmission side and an optical amplifier on the receive side share a pump light source.